1. Field of the Invention
This invention relates to a dual velocity strainer employed in fluid collection and distribution systems for ion exchange and filtration processes. In one aspect, this invention relates to a pipe mounted strainer which will permit different fluid velocities to transit through the strainer, dependent on the direction of fluid flow and which will prevent particulate matter from entering the fluid flow path.
2. Background Information
Properly designed ion exchange and media filtration beds employ means to suitably distribute the flow of a fluid into the bed and to collect the flow out of the bed in a manner that prevents dead spaces or pockets of poorly utilized ion exchange resin or filtration media. It is common in the industry to refer to a system which distributes or collects fluid in a vessel as a "distribution system". The remainder of this specification will use the common terminology of "distributor" or "distribution system" so as to avoid confusion, but it should be understood that a reference to a distributor or distribution system may make reference either to a fluid distribution or to a fluid collection system, or to both.
Laterals in a liquid distribution system include one or more horizontal conduits which are used to distribute a flow of liquid evenly over a plurality of openings in the conduit. The openings may include drilled holes in the conduit, or they may include a series of pipe fittings such as tees or elbows into which a plurality of strainers may be connected. The two designs most widely used for distribution and collection systems within a vessel comprising laterals are the hub-radial and the header-lateral types.
The hub-radial design utilizes a hollow central hub with hollow laterals connected to the hub in a radial fashion, like spokes in a wheel. Holes drilled in the laterals allow the fluid into or out of the laterals. The ends of the laterals are generally capped or plugged. The hub-radial design is generally limited to vessels having a diameter of 48 inches or less. If the hub-radial design is used in larger vessels, the distance between the diverging laterals near the vessel wall becomes too far to give an even distribution of the fluid being collected or for the fluid being distributed.
In order to overcome uneven distribution, some manufacturers of ion exchange and filtration equipment have installed a series of lateral distribution pipes extending perpendicularly from the radial distribution pipes in the same plane as, and near the ends of, the radial distribution pipes.
In order to keep ion exchange or media filter particles from entering the laterals, the laterals may be covered with screen, or they may be buried in a graded gravel bed.
Header-lateral distribution systems consist of a larger header pipe communicating with a plurality of smaller laterals. The header pipe generally enters the vessel at a right angle to the wall of the vessel at a location near the top or bottom of the vessel. The laterals communicate perpendicularly with the header pipe in a common plane which is parallel to the top or the bottom of the vessel in a vertically oriented vessel. The laterals may be constructed in a manner identical to the hub-radial laterals, or they may communicate with a plurality of vertical pipes which, in turn, communicate to a plurality of strainers.
Ion exchange and filtration vessels may be constructed as either spherical or cylindrical pressure vessels. The use of spherical vessels has been limited to high pressure applications where the wall thickness of a conventional cylindrical vessel would be prohibitive.
A spherical vessel utilizing both hub-radial and header lateral distribution is described in U.S. Pat. No. 3,063,565.
It is more common practice to construct ion exchange and filtration vessels with a cylindrical side wall and dished head ends. This practice is the most economical means to construct such vessels designed to withstand internal pressures of up to about 300 pounds per square inch above the external atmospheric pressure.
Many ion exchange systems are designed with a graded gravel subfill in the bottom dish to act as a support for the ion exchange or filtration media. Typical graded subfills consist of a 4" layer of 1/2".times.1/4" quartz, a 4" layer of 1/4".times.1/8" quartz, and a 4" layer of No. 4 quartz.
The distribution system may either be buried in the subfill, or it may lie just on the surface of the subfill. If the distribution system is buried in the 1/4".times.1/8" layer of the subfill, the holes in the distributors need not be screened to prevent ion exchange or filtration media from entering the distribution system.
If the distribution system lies on the surface of the subfill, a means must be provided to prevent the ion exchange or filtration media from entering the distribution system. Such means commonly comprise a plastic or metal screen material. The lateral pipes may also be enclosed with either a plastic or metal wellscreen material.
The distribution system is commonly connected to a vertical riser pipe which exits the bottom of the vessel and connects to the external piping and valves of the system.
A header-lateral distribution system utilizing strainer nozzles is described in U.S. Pat. No. 3,826,375.
In order to prevent stagnant areas in the subfill of ion exchange systems where regenerant chemicals may collect and possibly leach out during the subsequent service cycle, it is common practice to provide a series of weep holes in the vertical riser pipe to allow a slight flow of liquid through the gravel subfill which will gradually flush any residual chemicals from the subfill area.
A gravel subfill may be unacceptable in applications where leaching of chemicals from the gravel, such as silica, may prove detrimental to the process. High purity water applications for the semiconductor and power industries would preclude the use of a gravel subfill in ion exchange vessels. Gravel subfills are also undesirable in mixed bed ion exchange applications, where compressed air is introduced into the bottom of the bed for the purpose of mixing two or more types of ion exchange resins. Such agitation would disturb the graded gravel subfill, resulting in the entrainment of ion exchange resin in the subfill and the entrainment of gravel in the ion exchange bed.
In applications where gravel subfills are not desirable, a flat plate is constructed over the lower dished head, providing a "flat false bottom" to the vessel. Such a plate is usually supported on its underside by a series of concentric rings which are sized to provide support between the inside of the dished head and the underside of the false bottom.
U.S. Pat. No. 3,004,668 describes a vessel underdrain system comprised of a curved false tank bottom with a series of strainers protruding into the vessel from the false tank bottom.
Strainers constructed of substantially cylindrical well screen material are described more fully in U.S. Pat. No. 2,743,016.
Other strainers designed for mounting into bottom-plate supported beds are described in U.S. Pat. No. 3,472,382 and U.S. Pat. No. 3,730,348.
It is desirable for distribution systems to serve multiple functions. For instance, the underdrain system of an ion exchange bed should be capable of evenly collecting the flow of fluid during a service cycle. Such flows generally range between 8 and 20 gallons per minute for every one square foot of resin bed area and may go as high as 50 to 60 gallons per minute for every square foot of resin bed area. Service flows are generally in a downward direction, from the top of the bed to the bottom. The same underdrain system may be required to provide a reverse flow in the range of 4 to 5 gallons per minute per square foot of resin bed area during the backwash cycle, which is generally in an upward direction from the bottom of the bed to the top. This reverse flow through the bed is generally known as a backwash cycle.
The backwash cycle 1) removes dirt and broken or fine ion exchange particles; 2) removes any channeling or compaction of the bed, and, in the case of a mixed bed ion exchanger, 3) hydraulically separates the more dense cation exchange resin from the less dense anion exchange resin into two distinct layers so that the two resins may be regenerated with different chemicals.
It is also desirable for the underdrain system to collect the fluid during the service cycle at a low velocity, in order to prevent excessive pressure drop across the distribution system which would waste energy and may promote undesirable uneven channeling of the fluid through the ion exchange particles. However, the underdrain should provide a relatively high velocity fluid distribution flow path during the backwash cycle. This high velocity flow path should provide a planar scrubbing action across the junction of the ion exchange resin particles and the bottom of the vessel, in order to provide an even lifting motion which will backwash all of the resin particles.
Cation exchange resins are commonly regenerated with aqueous solutions of sulfuric acid, hydrochloric acid, sodium carbonate, or sodium chloride, depending on the particular resin and application. Other chemicals such as nitric acid, phosphoric acid, sodium hydroxide, potassium hydroxide, and ammonium hydroxide may also be used as regenerants, depending upon the application. Concentrations of these chemicals range from about 0.5% to about 15% by weight when used as regenerants for ion exchange resins. Anion exchange resins also can be regenerated with the above chemical solutions, depending upon the resin and application, but the most common anion exchange regenerants are sodium hydroxide and ammonium hydroxide solutions. Concentration ranges for the anion regenerants are essentially identical to the cation exchange regenerants. Flow rates for ion exchange resin regenerants typically range from two to five times less than the service flow rate through the same resin bed.
In the case of a mixed bed ion exchange bed, the underdrain system will also be used for the distribution of acid regenerant with a flow path countercurrent to the flow path during the service cycle. The flow of acid regenerant into the cation portion of the resin bed is generally at a much lower flow rate than the flow rate during the service cycle. The kinetics of the ion exchange process require this slower flow rate in order to achieve as complete a regeneration of the ion exchange resin as possible.
Many single resin ion exchange beds also utilize a regenerant flow which is countercurrent to the service flow. Regenerant flow rates in these systems are also generally much slower than the service flow rate. These systems claim a lower leakage of ions in subsequent service runs because the last portion of ion exchange resin to contact the liquid being purified has also been exposed to the largest portion of fresh regenerant chemical. In countercurrently regenerated systems, it is of particular importance for the relatively low flow rate of the regenerant solution to exit the distributor at a relatively high velocity in order to make intimate contact with all of the resin between the distributors. Any resin near the distributors which does not become completely regenerated will cause undesirable leakage of ions during the subsequent service cycle and will negate the advantages of the countercurrently regenerated ion exchange system.
In some cases, where the service flow rate and the backwash flow rate are very different, a separate service collector and backwash distributor are installed adjacent to each other, each communicating to external piping through a separate connection. Although such systems may be necessary to provide an even distribution or collection of liquid, they are more expensive than single distributor systems, and they are difficult to install and maintain.
Packed bed ion exchange systems may have a service cycle in which the flow of fluid is from the bottom of the bed to the top of the bed. The vessels utilized in these systems generally have both a flat false top and a flat false bottom. They may be packed completely with ion exchange resin or they may only be partially packed with ion exchange resin. In the latter case, the flow rate is usually high enough to hydraulically lift the bed and pack it against the top of the vessel as a plug. A small portion of the resin bed may remain fluidized. These same beds are generally regenerated in a direction opposite to the service flow direction, from the top of the bed to the bottom. In this case it would be desirable for the upper distributor also to operate in two velocity modes. The lower velocity mode would be adapted to the faster flow rate of the service cycle, and the higher velocity mode would be adapted to the lower flow rate of the regeneration cycle.
For these reasons, it is desirable for the distribution system in many ion exchange systems to have the dual capability of collecting a fluid at a low velocity and distributing a fluid at a higher velocity.
A detailed description of various types of fluid distribution and collection systems can be found in Chapter 5 of Practical Principles of Ion Exchange Water Treatment by Dean Owens, published by Tall Oaks Publishing, Inc., 1985.
U.S. Pat. No. 1,407,397 describes a strainer for a media filter with a flapper plate which is closed on a series of narrow channels during a downflow service cycle. A higher pressure drop is produced across the strainer during the service cycle, because of the narrow channel flow paths, resulting in even distribution of the flow among the plurality of strainers. When the filter is backwashed, generally at a higher flow rate than the service cycle, the reverse flow lifts up the flapper, exposing a larger flow path to the backwashing fluid. This strainer was designed for backwashing filters which utilize subfill and a header lateral distribution system and is operable only at the bottom of a vessel.
U.S. Pat. No. 4,162,975 discloses a dual velocity strainer. The disclosed system is preferably used with vessels which have a false bottom with perforations therein of the size adapted to accept the strainer. Retrofitting this strainer into existing equipment which does not have a perforated strainer plate would not be practical.